Optical system for a head mounted display using a retro-reflector and method of displaying an image

ABSTRACT

A head mounted display system includes focusing optics to focus light from an image source, retro-reflector and beamsplitter, the image source and beamsplitter directing light toward the retro-reflector being directed via the beamsplitter for viewing, thereby to use conjugate optics effectively to place the viewing eye or detector in relation to the image source the same as the relation of the focusing optics or it may be remotely located. A method for forming an image using conjugate optics which light from a source is focused to form a real image at a retro-reflector and is reflected for viewing.

TECHNICAL FIELD

The present invention relates generally, as is indicated, to optical displays, and more particularly, to head mounted displays and an optical system therefor using conjugate optics, for example, including a retro-reflector, and to a method of displaying an image.

BACKGROUND

Various types of head mounted displays are known. An exemplary head mounted display (throughout the following specification the initials "HMD" may be used to mean "head mounted display") includes optics or optical components such as lenses, mirrors or the like, to direct the image from an image source to an eye or to the respective eyes of a person viewing the image (viewer). The image source develops and/or provides an image intended to be viewed and may or may not be part of the HMD. Head mounted display systems are used in the field of virtual reality and also in aircraft, for example, as pan of a heads-up display system, and in other fields, too.

Many prior head mounted display systems use light emitting sources to create an image, such as a cathode ray tube, light emitting diode, etc. Several disadvantages to such light sources and head mounted displays using them are relatively large size, weight, and cumbersome nature. For example, in some virtual reality display systems, counterbalancing weights and support systems are needed to hold or to help to hold the helmet containing the virtual reality image source and optics so that it does not severely weigh down the head, neck, shoulders, etc. of the user.

In some prior display systems a modulator modulates light from a source; the images created are a function of modulation. A liquid crystal cell or liquid crystal display device may be such a modulator. A disadvantage of such modulating systems is the reduction in light output due to light blocking and/or absorption occurring in the modulator. To overcome such reduction in brightness, sometimes the intensity of the light source is increased, which draws additional energy, creates heat, requires a larger light source, etc.

Another disadvantage to prior head mounted display systems is the complexity of the components and of the arrangement of the components to provide the desired display or image output. Complexity, size, and so forth usually increase the cost for such systems and reduce the robustness of the system.

It is desirable that a display, especially a HMD, have adequate eye relief and comfort with which images provided by the HMD can be viewed. One aspect of comfort is the distance at which the image is viewed; a comfortable viewing distance is about twenty inches or more, for example, approximate reading distance. An aspect of eye relief is the distance between the eye and the last optical element (such as the output objective of the display) closest to the eye; often it is desirable that such distance be relatively large to provide adequate eye relief. If adequate eye relief is not provided, and/or if the viewing distance at which the image is seen is less than about twenty inches to the eye is too close, then the eye may be strained to view the image, which may be uncomfortable and usually is undesirable.

It would be desirable to reduce the size, weight and complexity of a display system, especially a head mounted display system.

It would be desirable to provide a relatively uncomplicated, small and robust display system, especially for a HMD.

It also would be desirable to provide a high quality image, e.g., bright and of good contrast and, if used, color, for viewing using a HMD, and especially to derive the image using a small display device.

Further, it would be desirable to obtain a relatively wide field of view in an optical display system, especially a head mounted one, and efficiently to deliver light produced by the light source to the viewer. Efficient delivery of light reduces the brightness requirement of the light source, energy requirements and output heat, while providing good brightness, resolution and contrast of the viewed image.

Additionally, it would be desirable to provide adequate eye relief in a head mounted optical display system.

SUMMARY

According to one aspect of the invention, a head mounted display system includes a retro-reflector, and optical means for directing light from an image source to the retro-reflector, light reflected by the retro-reflector being provided for viewing. The image source may be included as part of the HMD.

Briefly, according to the invention, light from an image source is directed by focusing optics to a conjugate optics path along which the light is directed to the eye or eyes of a viewer for viewing of an image.

In an embodiment of the invention the conjugate optics path is provided by one retro-reflector or more than one retro-reflector which at least substantially maintains the characteristics of the light incident thereon, including the results of the focusing by the focusing optics, while reflecting the light to the eye(s) of the viewer. In an embodiment of the invention a beamsplitter directs incident light from the focusing optics into the conjugate optics path, e.g., reflecting light or transmitting light with respect to the retro-reflector and to the eye(s) for viewing.

Also, light directed to the retro-reflector from the focusing optics mentioned above is reflected such that the light continues to have essentially or substantially the same direction it had when it impinged on the retro-reflector so that optically the lens of the eye can appear to be in effect at the focusing optics and the retina of the eye can appear to be in effect at the source of the image.

A further aspect is to use a relatively simple lens system to obtain a good angle of view and comfortable eye relief in a HMD.

Yet an additional aspect is to use a retro-reflector in a HMD, to focus light from an image source at a location relative to the retro-reflector, and to direct the light at least partly along the path incident to the retro-reflector for viewing of an image by a viewer, whereby a substantial amount of light from the image source is delivered to the eye of the viewer.

Even another aspect is to reduce the illumination requirements and/or size of the image source for a HMD by using an at least partially conjugate optical system to direct an image to the eyes of a viewer for viewing.

Even an additional aspect is to reduce the illumination requirements and/or size of the image source for a HMD by using an at least partially conjugate optical system to direct an image to the eyes of a viewer for viewing while providing comfortable eye relief.

Another aspect is to direct light having image characteristics from a retro-reflector to a viewer to provide the viewer an image of good brightness and resolution, such as a television or video display, for example, that is focused at a distance that provides comfortable viewing while providing acceptable eye relief.

Another aspect is to direct light having image characteristics from a retro-reflector to a viewer to provide the viewer an image that is focused at a distance that is approximately at on the order of about twenty inches, e.g., reading distance from the eye.

Another aspect is to direct light having image characteristics from a retro-reflector to a viewer to provide the viewer an image that is focused at a distance that is relatively easily focused by the viewer's eye without focusing at infinity.

According to another aspect, a display system includes an optical means for directing an image at a retro-reflector, and a beamsplitter for reflecting and transmitting light relative to the retro-reflector, whereby the beamsplitter one of transmits light and reflects light toward the retro-reflector and the other of transmits light and reflects light from the retro-reflector for viewing so that a real image is focused at a distance while comfortable eye relief is provided.

According to another aspect, a display system includes an optical means for directing an image at a retro-reflector, and a beamsplitter for reflecting and transmitting light relative to the retro-reflector, whereby the beamsplitter one of transmits light and reflects light toward the retro-reflector and the other of transmits light and reflects light from the retro-reflector for viewing so that a real image is focused at a distance that is on the order of about twenty inches from the eye.

According to another aspect, a display system includes an optical means for directing an image at a retro-reflector, and a beamsplitter for reflecting and transmitting light relative to the retro-reflector, whereby the beamsplitter one of transmits light and reflects light toward the retro-reflector and the other of transmits light and reflects light from the retro-reflector for viewing while maintaining adequate spacing between the last optical element of the display system most proximate the eye and the eye itself to provide comfortable eye relief.

Another aspect relates to using a retro-reflector or the like to collect light from a focusing optical system and to reflect that light for viewing especially for viewing an image, and, more particularly, for viewing of the image at a viewing distance that provides adequate eye relief for the viewer.

Another aspect is to provide a compact at least partly conjugate optical system for preparing an image for viewing at a viewing port such that the real image is at least a distance from the viewer's eyes to provide comfortable viewing at a distance of on the order of about twenty inches or more.

According to another aspect, a display system includes an optical means for focusing an image at or approximately at a retro-reflector or with respect to a retro-reflector, and a beamsplitter for reflecting and transmitting light relative to the retro-reflector, whereby the beamsplitter one of transmits light and reflects light toward the retro-reflector and the other of transmits light and reflects light from the retro-reflector for viewing.

According to another aspect, a display system includes means for forming an image for viewing by an eye of an observer, delivery means for delivering the image from the means for forming to the eye of the observer, and the means for forming and the delivery means being cooperative to provide the image to the eye of the observer at a size at the retina of the eye that is approximately the size of the retina.

According to another aspect, a method of display includes forming an image, and reflecting the image to the eye of an observer such that the image at the retina of the eye is approximately the size of the retina of the eye.

Another aspect is to reduce the size requirements for an image source used in a HMD.

Another aspect is to reduce the brightness requirement for an image source of an optical display, while providing a bright image for viewing.

An additional aspect is to magnify an image in a display, especially a HMD, without the need for an output objective.

According to another aspect, a display includes means for directing polarized light toward a reflector to form an image, and means for discriminating light as a function of polarization to direct light from the reflector for viewing.

According to another aspect, a display includes means for directing incident light having characteristics of an image toward a retro-reflector, means for permitting viewing of reflected light reflected by the retro-reflector as a focused image, the means for permitting viewing including means for reflecting at least some of one of the incident light and reflected light and for transmitting the other of the incident and reflected light.

According to another aspect, a method of displaying an image includes directing in a first direction incident light having image characteristics toward a retro-reflector for reflection thereby, directing in a generally parallel and opposite direction to the first direction reflected light from the retro-reflector for viewing, and directing in a direction that is not at least generally parallel to the first direction at least one of the incident light prior to being directed in the first direction and the reflected light subsequent to being directed in the generally parallel opposite direction to the first direction.

According to a further aspect, a head mounted display is relatively compact, light weight, robust and able to provide relatively bright images at a viewing distance from a viewer's eye that provides comfortable viewing and which also provides eye relief.

One or more of these and other objects, features and advantages of the present invention are accomplished using the invention described and claimed below.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

Although the invention is shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

FIG. 1 is a schematic illustration of a head mounted display (HMD) optical system in accordance with the present invention shown mounted on the head of a person relative to one of the eyes;

FIG. 2 is a schematic illustration of an HMD optical system utilizing a retro-reflector in accordance with the invention;

FIG. 3 is a schematic illustration of the HMD optical system showing the relationship of the size of the image on the retina of the viewing eye relative to the object to image distance;

FIG. 4 is a simplified schematic illustration of an HMD optical system similar to FIG. 2 using a linear polarizing beamsplitter;

FIG. 5 is a simplified schematic illustration of an HMD optical system similar to FIG. 2 using a cholesteric liquid crystal beamsplitter;

FIG. 6 is a simplified schematic illustration of an HMD optical system similar to FIG. 2 showing a comfortable viewing distance for the eye;

FIG. 7 is a schematic illustration of a modified HMD optical system similar to FIG. 2 using an additional lens component to obtain comfortable viewing distance while reducing the actual size of the system;

FIG. 8 is a schematic illustration of an HMD optical system similar to FIG. 2 including a light source;

FIG. 9 is a schematic illustration of an HMD optical system similar to FIG. 8 including an example of a shared light source for two display devices used in a system according to the invention;

FIG. 10 is a schematic illustration of another embodiment of HMD using plural retro-reflectors respectively, positioned relative to the beamsplitter; and

FIG. 11 is a schematic illustration of HMD using a multiple part retro-reflector.

DESCRIPTION

Referring to the drawings in which like reference numerals designate like parts in the several figures and primed reference numerals and reference numerals with a letter suffix may represent parts which are similar to other parts designated by the same reference numeral, as is evident from the drawings and descriptive text of the specification, and initially to FIG. 1, a head mounted display system (referred to below as HMD) according to the invention is shown at 10. The HMD 10 includes a housing 11 and a support 12. The HMD is intended to be mounted with respect to a person who can view images provided by the HMD. An exemplary eye 13 of a person is shown in FIG. 1 positioned relative to the HMD 10. The HMD may have two optical portions (only one being shown in FIG. 1 for simplicity of illustration), each being similar to that shown in the several drawings, for respectively providing images to the respective eyes of a person. The images may be coordinated to provide a three dimensional (3-D) viewing effect, a two dimensional (planar) viewing effect, or some other effect. Alternatively, one device, such as that shown in FIGS. 1-8, 10 and/or 11, for example, may provide images to both eyes of a person. The HMD 10 also may be used to provide images to other devices, such as cameras (still, motion, video, electronic, etc.), measuring equipment, optical evaluation equipment, copiers, scanners, etc. For illustrative purposes the HMD will be described in detail with respect to use to provide images for viewing by a person.

The HMD 10 preferably is relatively small and lightweight to facilitate positioning relative to a person, and especially to facilitate positioning relative to the head of a person to provide images to the eyes. In the embodiment illustrated in FIG. 1 the housing 11 is mounted by the support 12 to the head of a person whose eye 13 is depicted schematically. The support 12 may include a strap, cap, temple piece as in eye glasses, or other device to mount the housing 11 on the head or body of a person or relative thereto to position the housing 11 in front of the eyes. The support 12 may be part of or may be attached to another support structure mounted on a wall, floor, ceiling, or other structure of a building, vehicle, etc.

The housing 11 contains one or more of the components of the optical system 14 of the HMD 10, as will be described in greater detail below. The optical system 14 may include, as is designated at 15, for example, a light source and/or source of images to be provided by the HMD for viewing, or the housing and optical system may be coupled to another source, for example, an external source, such as a liquid crystal display, a cathode ray tube display (CRT), a television, or some other source, which supplies light and/or images. The housing 11 may completely or partially enclose the various optical components of the HMD 10. The housing has an opening or outlet as a viewing port 16 through which the eye 13 can view images provided by the HMD. Alternatively, the housing 11 may have one or more additional openings to receive light and/or a source of images, etc., for delivery ultimately to the viewing port 16. As a further alternative, the housing 11 may be one or several structural members which support but do not necessarily enclose one or more optical components of the HMD so that those members are in relative position to provide images for viewing.

In FIG. 2 details of optical components of the optical system 14 of the HMD 10 are shown. The optical components shown in FIG. 2 are similar to those included in the housing 11 of FIG. 1; however, in FIG. 2 the housing 11 and support 12 are not shown to facilitate illustrating the invention and to simplify the drawing.

The optical components 20 of the optical system 14 of the HMD 10 include focusing optics 21, a beamsplitter 22 and retro-reflector 23. The HMD also may include an image source 24 which provides images or light having characteristics of an image. An exemplary image source is a liquid crystal display, such as a small liquid crystal television having a cross-sectional display area on the order of about one square inch or less. Alternatively, the image source may be separate and simply used to provide one or more images or light having image characteristics that can be provided by the HMD to the eye 13. Additional optical components of the optical system 14 may include linear polarizers, circular polarizers, waveplates, focusing elements, such as lenses or mirrors, prisms, filters, shutters, apertures, diaphragms, and/or other components that may be used to provide a particular type of output image for viewing by the eye 13. Examples of several embodiments using such additional optical components are described below with respect to other drawing figures.

The invention is useful with virtually any type of image source or display source. An example of such a display source is a compact flat panel display, and especially one utilizing a reflective liquid crystal display made from a single crystal silicon active matrix array. An example of such image source is disclosed in copending, commonly owned U.S. patent application Ser. No. 08/275,907, filed Jul. 5, 1994, the entire disclosure of which hereby is incorporated by reference.

In FIG. 2 the image source 24 displays an image 25, which is shown in the drawing as an arrow 26. The light 27 leaving the image source 24 represents an image or has characteristics of an image, and that light is collected by the focusing optics 21 of the optical system 14 of the HMD 10 and travels to the beamsplitter 22. In FIG. 2 and in a number of the other drawing figures hereof the focusing optics 21 is represented as a single lens. However, it will be appreciated that the focusing optics 21 may include one or more other components, such as lenses, reflectors, filters, polarizers, waveplates, etc.

At least some of the light 27a incident on the beamsplitter 22 is reflected by the beamsplitter as light 27b toward the retro-reflector 23. The retro-reflector may be, for example, a screen made of retro-reflecting material. Exemplary retro-reflectors are well known. One example is that known as a comer reflector or a sheet having a plurality of comer reflectors. Another example is a material having plural glass beads or other refracting and/or reflecting devices on or in a support. An example of a retro-reflector is a film or sheet material having a plurality of corner cubes which material is sold by Reflexite Corporation of New Britain, Conn. Such material is available having about forty-seven thousand corner reflectors per square inch.

The light (light rays) 27c, which are shown as broken lines, are reflected by the retro-reflector 23 such that their path is exactly back along their direction of incidence on the retro-reflector. In this way the light rays 27c pass through the beamsplitter 22 and are focused, i.e., so to present a real image in focus as seen by the light ray diagram of FIG. 2, at a point or location in space, which is generally designated 28 in the illustration of FIG. 2. The eye 13 of a viewer (person) can be placed approximately at location 28 to see the image, and the pupil 29a and lens, 29b individually and collectively also designated 29, of the eye, accordingly, are shown at that point. The lens 29 focuses the light incident thereon as an image on the retina of the eye 13.

In FIG. 2 the broken lines represent light rays which travel after reflection by the retro-reflector along the same or substantially the same path, but in the opposite direction to, respective incident light rays impinging on the retro-reflector. Thus, the retro-reflector 23 is part of a conjugate optics path 23a in which light incident thereon is reflected in the same path and opposite direction as reflected light. The beamsplitter 22 directs light from the focusing optics 21 into that conjugate optics path and toward the retro-reflector; and the beamsplitter also passes light in the conjugate optics path from the retro-reflector to the output port 16 (FIG. 1) for viewing by the eye 13. The beamsplitter 22 and retro-reflector 23 cooperate as a conjugate optics system to provide that conjugate optics path.

Advantageously using the described conjugate optics path and system, relatively minimal amount of the light from the image source 24 and focusing optics 21 is lost and, conversely, relatively maximum amount of light is directed to the eye 13. Also, there is substantial accuracy of image and image resolution conveyed to the eye. Furthermore, especially if a relatively good quality retro-reflector is used so that the precise location at which the image 30 is in focus will not be critical, e.g., it can be behind or in front of the retro-reflector, there is a reduced tolerance required for the relative positioning of the components of the optical system 14. This makes the HMD 10 relatively robust and reliable.

In FIG. 2 the viewed image 30 is represented by an enlarged arrow 31. Such arrow 31 is shown in FIG. 2 as a magnified focused image of the image 25 from the image source 24. The image 30 may be in focus at or approximately at the retro-reflector 23, and this is especially desirable for good quality images to be provided the eye 13 when a relatively low quality retro-reflector is used. A low quality retro-reflector is one which has relatively low resolution or accuracy of reflecting light in a conjugate manner in the same path but opposite direction relative to the incident light. With a low or poor quality retro-reflector and the image not being focused at the retro-reflector, it is possible that too much light may be lost from the desired conjugate optics path back to the eye 13, and this can reduce the quality of the image seen. However, the image 30 may be in focus at another location or plane either behind the retro-reflector (relative to the eye) or in front of the retro-reflector, and this is easier to do while maintaining a good quality image for viewing when the retro-reflector is a good quality one. The better the retro-reflector, the more self-conjugating is the optical system 14 and the less the need to focus with precision at the retro-reflector.

In considering the brightness of the image seen by the viewer, the nature of the beamsplitter 22 plays a role. The light produced by the image source 24 may be polarized or unpolarized. If the beamsplitter 22 is of a non-polarizing type, then a balanced situation is to have 50% of the light incident on the beamsplitter 22 be reflected and 50% transmitted. Thus, of the light 27a on incident the beamsplitter 22, 50% is reflected and sent toward the retro-reflector screen 23 as light 2b. Of the reflected light 27c from the retro-reflector 23, 50% of the light will be transmitted through the beamsplitter 22 and will travel to the viewer's eye 13. This configuration of the optical components 20 of the HMD 10 can transfer to the viewer's eye a maximum of 25% of the light produced by the image source 24. If desired, the beamsplitter 22 can be modified in ways that are well known to change the ratio of the reflected light to transmitted light thereby. Also, the beamsplitter 22 may include an anti-reflection coating so that all or an increased amount of the image comes from one side of the beamsplitter and thus to reduce the likelihood of a double image.

Also, since the optical system 14 of the HMD 10 provides good resolution of the image and maintains the characteristics thereof, the image source can be a relatively inexpensive one that does not have to compensate for substantial loss of image quality that may occur in prior HMD systems. Furthermore, since a relatively large amount of the light provided by the image source 24 is provided to the eye 13 for viewing, e.g., since the retro-reflector can virtually focus the light at the eye, additional brightness compensation for loss of light, as may be needed in prior HMD systems, ordinarily would not be required.

For exemplary purposes, in FIG. 2 three light rays 40a, 40b, 40c (collectively 40) originating at the tip of the arrow 26 constitute a portion of the light 27. Three light rays schematically shown at 41a, 41b, 41c (collectively 41) also are examples of light emanating at the tail of the arrow 26. The light 27 has characteristics of the image 25 from or provided by or at the image source 24, and represented by the exemplary light rays 40 and 41, is focused by the focusing optics 21 onto the retro-reflector 23. The size of the image 30 seen as the arrow 31 on the retro-reflector 23 depends on the focal length of the focusing optics 21 and the distances between the image source 24 and the retro-reflector 23 from the focal points 43, 44 of the focusing optics 21. The image source 24 should be located relative to the focusing optics 21 such that an image can be focused, e.g., in focus as is shown in FIG. 2, at or approximately at the retro-reflector. For example, the image source 24 may be beyond the focal point 43 of the focusing optics 21, and the retro-reflector likewise preferably is beyond the focal point 44 of the focusing optics so that the image can be focused at the retro-reflector.

In the illustration of FIG. 2 the image 30 on the retro-reflector 23 is magnified relative to the size of the image at the image source 24; it does not have to be magnified. The image 30 may be the same size as the image 25 or it may be smaller. Thus, although the image source 24 may be relatively small and/or may provide a relatively small size image 25 at its output, the size of the image 30 viewed by the eye 13 may be different.

Turning to FIG. 3, the arrangement of optical components in the optical system 14 of the HMD 10 is presented to demonstrate the obtaining of a wide field of view in accordance with the invention. As is seen in FIG. 3, exemplary light rays 40b, 41b originate from the respective ends of the image 25 arrow 26 at the image source 24, and light ray 50 originates from the center of the arrow 26. These light rays 27a pass through the focusing optics 21, here shown as a lens, are reflected by the beamsplitter 22, and are incident on the retro-reflector screen 23 as light 2b. The light rays are retro-reflected by the retro-reflector screen 23 as light 27c so that they converge, i.e., are focused, at a point 28 in space. As is seen in FIG. 3, when the pupil 29(a) of a viewer's eye 13 is placed at this point 28 in space lens 29b focuses the image onto the retina and it is possible to see the image 30.

To obtain a relatively wide field of view the entire retina 51 of the eye 13 may be filled with the image, shown at 52 in FIG. 3. The more the retina is filled, the wider will be the field of view. In the illustrated HMD 10 the size of the image 52 on the retina 51 equals the object distance between the image source 24 and the focusing optics 21, for example, which is shown at 52 in FIG. 3, divided by the image distance shown at 54. The image distance, of course, is a function of the distance between the lens 55 of the eye 13 and the retina 51.

Since the size of the retina is on the order of about one square inch, the largest image put on the retina is also about one inch. As can be seen in FIG. 3, this implies that, by a proper choice of lens f# (f number) and object distance 53, it is possible to fill the pupil 29 of the eye while utilizing an image source 24 that is also no larger than about one square inch. Therefore, a relatively small image source 24 may be used in the invention while obtaining a relatively wide field of view. Also, by using the magnification described above, a relatively small size image source, say less than about one square inch in display area, can provide sufficiently large area image at the eye for viewing.

Also, in the arrangement of optical components 20 of the HMD as illustrated in FIG. 3, the pupil 29a, of the eye 13 is located at a distance from the beamsplitter 22. This distance, which is usually considered in relation to "eye relief", also is determined by the f# of the focusing optics 21. By adjusting the parameters, such as f# and object distance, the eye relief can be made at any distance that is comfortable and convenient.

In operation of the HMD 10, light having characteristics of an image is provided by the image source 24. The focusing optics 21 directs the light to the beamsplitter 22, which reflects the light to the retro-reflector 23 for focusing approximately at or near the retro-reflector. Light is reflected by the retro-reflector to transmit through the beamsplitter and to reach the eye 13. The eye lens 29 focuses the light onto the retina 51 to see the image.

The back of the eye 13, e.g., the retina, in effect is at the image source 24 relative to the focusing optics 21, and the lens 29 of the eye in effect is at the focusing optics relative to the image source. Thus, the lens 29 of the eye stands relative to the image source 24 in the same relation as the lens of the focusing optics 21 stands relative to the image source. In other words, as is illustrated in FIGS. 1-3, for example, and also in other figures hereof, the display 10 and the viewer's eye 13 are relatively positioned such that the focusing optics and beamsplitter are in substantially the same relation to each other as the pupil of the viewer's eye stands in relation to the beamsplitter to view an image from the image source when light is reflected in the conjugate optical path. Therefore, as is illustrated and functionally described herein, the focusing optics 21 and the eye lens 29 are in mirror image relation to each other relative to the beamsplitter 22. One has a full field of view as provided by the focusing optics. The location of the image seen by the eye 13 is a function of the plane at which the focusing optics focuses its projected image.

According to the invention, the HMD uses a retro-reflector, and the focusing optics 21 focuses the image from the image source at or approximately at the retro-reflector. Thus, a large amount, for example, substantially all the light from the focusing optics, can be directed by the retro-reflector and put at the eye 13. Of course, there may be other losses in the various components of the optical system, and there may be loss of light due to transmission of the incident light from the focusing optics onto the beamsplitter; and there may be reflection of light from the retro-reflector by the beamsplitter back toward the focusing optics and image source. Some of these losses are addressed in the several embodiments described further below. However, in view of the efficiency of the optical system 14 of the invention, relatively low illumination level may be required from the image source.

Using the invention it is possible to magnify the image from a relatively small display to cream a suitable image for viewing without having to place the optics and/or image source too close to the eyes, and, therefore, comfortable eye relief can be obtained. Furthermore, using a relatively simple and uncomplicated focusing optics 21, such as a single lens or a camera lens system, such as that used in a conventional SLR camera, it is possible to obtain a relatively good angle of view and good, comfortable eye relief. Using the invention it may be unnecessary to use an output objective or other objective that is placed rather close to the eye of the viewer while still providing an acceptable image for viewing.

For some magnification situations, as is described further in detail below, it is desirable to use a relatively short focal length focusing optics 21 such that the focal length thereof is less than that of the lens 29 of the eye 13 to the back of the eye. Also, to provide a relatively large or wide "sweet spot" or place where the eye 13 can be positioned relative to the optical system 14 and/or the output port 16, while still being able to see a good quality (bright, good resolution, and/or good contrast, etc.) image preferably also with a relatively wide field of view, it is desirable to use a relatively short focal length lens or focusing optics 21, and even more preferably to use such a focusing optics 21, indeed, optical system 14 overall, which has a relatively low f#.

In FIG. 4 is a modified HMD 10 which includes an image source 24' that produces light that is linearly polarized (sometimes referred to as plane polarized). This actually is the case in many liquid crystal based image sources and is the case using the image source of the above-referenced copending U.S. patent application. Using an image source that produces linearly polarized light, it is possible to improve the efficiency of the illumination system of the HMD 10. Such improved efficiency system uses a beamsplitter 22' which reflects light having one linear polarization (direction of plane polarization) but transmits the other direction of linear polarization without attenuation. Such a beamsplitter 22' is known and can be made, for example, by depositing thin films on a clear substrate.

The light 27' produced by the image source 24 is linearly polarized at right angles to the plane of the page. The beamsplitter 22' is designed to reflect this polarization so that most, preferably all, of the incident light 27a is reflected as light 27b toward the retro-reflector 23. In front of the retro-reflector 23 is placed a quarter waveplate 60. Preferably the quarter waveplate is oriented such that the axis thereof is at 45 degrees to the plane of polarization of the linearly polarized light 27b incident thereon. However, if desired, the orientation of the quarter waveplate may be different.

The quarter waveplate 60 converts the linearly polarized light 27b into circularly polarized light 27b' of a given handedness. When this light is reflected by the retro-reflector 23 as light 27c', it will be converted into circularly polarized light of the other handedness. When the light 27c' passes back through the quarter waveplate 60, such light is converted from circularly polarized light back into linearly polarized light 27c, but, this time, the axis or plane of polarization will be in the plane of the page, i.e., relatively orthogonal or perpendicular to the plane of polarization of the linearly polarized light 27b.

The light 27c with such linear polarization will be transmitted, preferably all of it will be transmitted, by the polarizing beamsplitter 22'. The light transmitted through the polarizing beamsplitter 22' then travels to the viewer's eye 13. It will be appreciated that the polarizing system, polarizing efficiency, etc., of the various elements and light in the HMD 10 shown in FIG. 4 ordinarily will not be 100% nor will the conversions between linearly polarized light and circularly polarized light be perfect. These non-idealities reduce the actual percentage of light transmitted to the viewer.

FIG. 5 illustrates a variation of the HMD 10 which uses an image source 24' that develops linearly polarized light and a wave plate 62, such as a quarter wave plate, that is oriented with its axis at 45° to the plane of polarization thereby to convert the plane polarized light from the image source to circularly polarized light of a particular handedness (direction, e.g., left or right circularly polarized light). Alternatively, the image source may produce light which is circularly polarized, and in this case it may not be necessary to use a wave plate. The focusing optics 21 directs the circularly polarized light 27' onto the beamsplitter 22". The beamsplitter 22" reflects one circularly polarized handedness light and transmits the other handedness. An example of such a beamsplitter is one which uses cholesteric liquid crystal material, as is known.

The beamsplitter 22" reflects light of the incident handedness from the focusing optics onto the retro-reflector 23; and upon reflection by the retro-reflector, the direction of circular polarization is reversed. Therefore, light incident on the beamsplitter 22" from the retro-reflector 23 transmits through the beamsplitter to the eye 13. Because a cholesteric beamsplitter can be a more efficient polarizer than a thin film beamsplitter, it is possible that the cholesteric based optical system of the HMD in FIG. 5 will deliver more light to the viewer than the HMD 10 of FIG. 2, for example.

In the above description several examples of beamsplitters were presented. It will be appreciated that other types of beamsplitters also may be used. One example, is a beamsplitter which only reflects light at some parts and transmits light at other parts.

Furthermore, it will be appreciated that although the invention is described and illustrated herein as having the beamsplitter reflect light from the image source to the retro-reflector; and then the beamsplitter transmits light from the retro-reflector to the eye, other arrangements of the optical components also are possible. An example of another such arrangement would have the image source, beamsplitter and retro-reflector in a straight line so the beamsplitter transmits light from the image source and focusing optics to the retro-reflector; and light that is reflected by the retro-reflector is reflected by the beamsplitter to the eye for viewing.

The human eye is most comfortable when viewing an image at a distance of about twenty inches, approximately at the distance at which one would place a book, document, etc. to be read. It is desirable that the final image as seen by the viewer be located at such distance, e.g., approximately twenty inches from the pupil 29 of the eye. This can be accomplished in the manner illustrated in FIG. 6. The retro-reflector 23 on which the focused image from the image source 24 is moved to twenty inches from the pupil 29 of the eye 13. A disadvantage with this system, though, is the large size of the HMD 10 required to provide such distance between the eye and the retro-reflector.

The HMD can be made more compact compared to the HMD illustrated in FIG. 6 by adding an additional optical system 70 between the beamsplitter 22 and the eye 13. In FIG. 7 such optical system 70 is depicted as a single lens; however, it will be appreciated that it may include other optical components as was mentioned above, for example, with respect to the focusing optics 21. In the HMD 10 of FIG. 7 the retro-reflector 23 is closer to the eye 13 than it is in FIG. 6 making a more compact system. The viewer is provided with a virtual image 30 of the image source 24 at the desired viewing distance (twenty inches, for example) by the cooperation between the focusing optics, retro-reflector, and additional optical system 70.

FIGS. 8 and 9 depict means to provide light to illuminate a reflective image source 24. The light source 75 can be of any type including multiple light emitting diodes. The configuration illustrated in FIG. 8 has the light source located in the plane of the page as are all other components of the optical system 14. Light from the source 75 is reflected by an additional beamsplitter 76 to the image source 24. Light from the image source 24 is transmitted through the focusing optics 21 and additional beamsplitter 76 to the beamsplitter 22. The remaining operation of the retro-reflector 23, beamsplitter 22 and eye 13 are as was described above with respect to the above embodiments, for example. In FIG. 9 the light source 75 is rotated relative to the orientation shown in FIG. 8, so that the light source can provide illumination of two respective transmissive, as shown image sources 24 via the illustrated respective beamsplitter 78 and reflector 79 and respective optical systems 14 in a two unit HMD for viewing of respective images simultaneously by both eyes of a person.

FIG. 10 illustrates another embodiment of HMD 100, features of which can be used with the several embodiments described above. In particular, an additional retro-reflector 23' is added at an orientation and location relative to the beamsplitter 22 and the original retro-reflector 23 such that the additional retro-reflector reflects light from the image source that previously was lost to the optical system 14. Specifically, light from the focusing optics 21 and image source 24 is reflected by the beamsplitter 22 to the retro-reflector 23, and the retro-reflector 23 relects to the beamsplitter for transmission to the eye 13. Additionally, light from the focusing optics 21 which is transmitted through the beamsplitter 22 to the additional retro-reflector 23' is reflected by the additional retro-reflector 23' back to the beamsplitter 22 for reflection to the eye 13. Although some light from the retro-reflector 23 may be reflected by the beamsplitter back to the image source 24 and some light from the additional retro-reflector 23' may be transmitted through the beamsplitter to the image source 24, such light is not necessarily lost to the optical system 14 of the HMD 100. Rather, such light may be used to increase the brightness of the light incident on the image source when such source is a reflective one, and, thus, further increase the brightness of the image viewed by the eye 13.

It will be appreciated that the HMD 100 increases the amount of light to the viewer, and, thus, increases the brightness of the output image while minimizing the illumination requirements of the optical system 14.

In some instances it is possible that the retro-reflector may not be perfectly flat or that it is not sufficiently large for the HMD. It has been found that the orientation of the retro-reflector 23 in the optical systems of the several embodiments described and illustrated may be other than flat and/or may be in multiple parts. Moreover, the parts need not be perfectly flat or parallel; rather the several parts can be in different orientations, provided the orientations are sufficient to provide the desired retro-reflection function described herein. An example of such non-parallel or linear orientation of a retro-reflector 23a, 23b is illustrated in FIG. 11. An HMD 10 using such multiple part retro-reflector, 23a, 23b without regard to whether the retro-reflector is flat or the parts thereof are parallel, has been found to be functional in the manner described above to provide images for viewing by the eye 13. 

I claim:
 1. A head mounted display for presenting an image for viewing by a viewer, comprisinga mounting structure mountable on the head of a viewer, a liquid crystal display image source carried by the mounting structure and providing an image output for viewing by one eye of a viewer, focusing optics carried by the mounting structure in position to receive light from the image source to form a real image, a retro-reflector having a characteristic of providing a conjugate optical path leading to said eye of a viewer, a beamsplitter carried by the mounting structure in position to receive light from the focusing optics and image source and to direct the light toward said retro-reflector, and said mounting structure including means for holding said beamsplitter and focusing optics with respect to the head of a viewer to orient said focusing optics and said beamsplitter with respect to said eye of the viewer such that said focusing optics and said beamsplitter are in substantially the same relation to each other as the pupil of said eye of a viewer stands in relation to said beamsplitter to permit said eye to view an image from said image source when light representing that image is reflected in said conjugate optical path.
 2. The display of claim 1, said mounting structure comprising head gear for mounting on the head of a viewer.
 3. The display of claim 2, wherein the size of the display area of the image source is about the size of the image on the retina of said eye of the viewer.
 4. The display of claim 1, wherein said retro-reflector is in said conjugate optical path receiving light from said beamsplitter and reflecting light back toward said beamsplitter.
 5. The display of claim 4, comprising a further retro-reflector in a further conjugate optical path receiving light from said beamsplitter and reflecting light back to said beamsplitter, said retro-reflector being positioned to reflect in said conjugate optical path light directed to said retro-reflector by reflection from said beamsplitter and said further retro-reflector being positioned to reflect in said further conjugate optical path light directed to said further retro-reflector by transmission through said beamsplitter.
 6. The display of claim 1, said focusing optics having an input to receive light from said image source and wherein said image source, said beamsplitter and said focusing optics are cooperatively related effectively to place the lens of the viewer's eye at said input to said focusing optics receiving light from said image source.
 7. The display of claim 6, said focusing optics having an input to receive light from said image source, and wherein the entrance pupil of the viewer's eye forms the exit pupil of the display.
 8. The display of claim 1, wherein said retro-reflector is positioned to reflect in said conjugate optical path light directed to said retro-reflector by reflection from said beamsplitter.
 9. The display of claim 8, wherein said beamsplitter is positioned relative to the conjugating optical path to direct light received from the retro-reflector toward a viewer for viewing.
 10. The display of claim 1, comprising a further image source, said image source and further image source being operative to present to the respective eyes of a viewer respective images.
 11. The display of claim 1, comprising a further image source cooperative with said image source to present to the respective eyes of a viewer respective images to form a stereoscopic view.
 12. The display of claim 11, further comprising in optical relation to said further image source, a further beamsplitter, and further focusing optics, andsaid means for holding including means for holding said further image source, said further beamsplitter and said further focusing optics with respect to the head of a viewer to orient said further focusing optics and said further beamsplitter with respect to the other eye of the viewer such that said further focusing optics and said further beamsplitter are in substantially the same relation to each other as the pupil of such other eye of a viewer stands in relation to said further beamsplitter to view an image from said further image source when light is reflected in a respective conjugate optical path including a portion leading to such other eye.
 13. The display of claim 1, comprising a further image source to present an image for viewing by a viewer, said image source and said further image source producing respective images, and said means for holding comprising means for holding each of said image sources in fixed relationship with a respective eye of a viewer to view an image from the respective image source.
 14. The display of claim 1, wherein said focusing optics and the eye of a viewer are positionally related to each other along said conjugate optical path in mirror image relation to said beamsplitter.
 15. The display of claim 1, wherein said focusing optics and retro-reflector are cooperative to place said image source and retina of the eye of the viewer in conjugate relation.
 16. The display of claim 15, wherein the display area of the image source and the size of the area of the image on the retina of the eye of the viewer are about the same size.
 17. The display of claim 15, wherein the display area of the image source and the focusing optics are cooperatively related to provide an image on the retina of the eye of the viewer that substantially fills the retina.
 18. The display of claim 1, wherein said focusing optics and retro-reflector are cooperative to place said focusing optics and the pupil of the eye of the viewer in conjugate relation.
 19. The display of claim 18, wherein the focal length of the focusing optics is substantially the same size as the focal length of the eye.
 20. The display of claim 1, wherein said focusing optics comprises means to focus light from the image source to form a real image with respect to the retro-reflector.
 21. The display of claim 1, wherein the focusing optics, beamsplitter and conjugate optical path are cooperative to present an image at a viewing location for viewing by the eye of a viewer such that the lens of the eye at the viewing location stands relative to the image source in the same relation as the focusing optics stands relative to the image source.
 22. The display of claim 1, wherein said beamsplitter comprises a cholesteric liquid crystal material for reflecting light having one direction of circular polarization and for transmitting light having a different direction of circular polarization.
 23. The display of claim 22, further comprising means for providing from the image source circularly polarized light, and wherein said retro-reflector is operative to reverse the direction of circular polarization of circularly polarized light incident thereon, and further comprising means for providing from the image source circularly polarized light, and wherein said retro-reflector is operative to reverse the direction of circular polarization of circularly polarized light incident thereon.
 24. The display of claim 1, wherein said image source includes means to provide an image output that has optical polarization characteristic, and said beamsplitter comprises a polarizing beamsplitter cooperative with the polarization characteristic of the image source to reflect light having one polarization characteristic and to transmit light having the opposite polarization characteristic.
 25. The display of claim 23, wherein light provided to the retro-reflector is plane polarized for reflection or transmission by the beamsplitter, and further comprising a quarter wave plate in the optical path between the beamsplitter and retro-reflector oriented to convert plane polarized light to circularly polarized light, and wherein said retro-reflector reverses the direction of circular polarization of circularly polarized light incident thereon, whereby upon transmission through the quarter wave plate such reflected light is converted to plane polarized light having a different direction plane of polarization than the light initially provided to the retro-reflector.
 26. The display of claim 1, further comprising an optical system that forms a virtual image of light in the conjugate optical path for viewing at the viewing location.
 27. The display of claim 26, said optical system comprising a lens.
 28. The display of claim 1, said means for holding comprising a housing for at least partly containing said beamsplitter and focusing optics.
 29. The display of claim 28, said means for holding comprising means for mounting the image source with respect to the housing.
 30. The display of claim 29, said means for holding comprising a support structure supporting the housing in position for relative placement with respect to the head of a viewer to provide respective images to the eyes of the viewer.
 31. The display of claim 28, said means for holding further comprising a support for mounting the housing on the head of a viewer.
 32. The display of claim 1, wherein said retro-reflector is positioned to reflect in said conjugate optical path light directed to said retro-reflector by transmission through said beamsplitter.
 33. A method for viewing an image, comprisingdirecting light from a liquid crystal display image source for viewing by one eye of a viewer, the image source being mountable with respect to the head of a viewer, said directing including directing light from the liquid crystal display image source via focusing optics and a beamsplitter toward a retro-reflector which has a characteristic of reflecting incident light along a conjugate light path, said directing including forming a real image, reflecting light by the retro-reflector along such a conjugate light path for viewing by a viewer, and positioning the pupil of the eye of a viewer to stand in the same relation to the beamsplitter as the focusing optics stands in relation to the beamsplitter to view an image from the image source when light is reflected in the conjugate light path.
 34. The method of claim 33, said directing including directing light from the focusing optics via the beamsplitter to the retro-reflector and said reflecting light comprising directing light reflected by the retro-reflector via the beamsplitter to the eye for viewing.
 35. The method of claim 33, said directing comprising presenting such image for viewing by one eye of a viewer and presenting an image to the other eye of the viewer, including directing light from a further image source via focusing optics in a further conjugate optical path in which the further image source and the other eye of the viewer are in conjugate relationship to view an image from the further image source when light is reflected in the further conjugate optical path.
 36. The method of claim 33, said directing comprising presenting such image for viewing by one eye of a viewer and presenting an image to the other eye of the viewer to provide stereoscopic view, including directing light from a further image source via focusing optics in a further conjugate optical path in which the further image source and the other eye of the viewer are in conjugate relationship to view an image from the further image source when light is reflected in the further conjugate optical path.
 37. The method of claim 33, said positioning comprising mounting the focusing optics and beamsplitter with respect to the head of the viewer.
 38. The method of claim 37, said positioning comprising mounting the focusing optics and beamsplitter on the head of the viewer as a head mounted display.
 39. The method of claim 33, said positioning comprising mounting the focusing optics, beamsplitter and image source with respect to a housing having a viewing outlet at which a viewer may view an image.
 40. A method for viewing an image, comprisingdirecting light from liquid crystal display image source for viewing by one eye of a viewer, the image source being mountable with respect to the head of a viewer, said directing including directing the light from the liquid crystal display image source via focusing optics and a beamsplitter to form a real image, the focusing optics and beamsplitter being positioned in relation to each other and said directing light comprising directing light along a conjugate light path, reflecting light along the conjugate light path via the beamsplitter to the eye of a viewer, said directing light via focusing optics comprising directing light via focusing optics which has a focal length about the same or smaller than the focal length of the eye of the viewer, and positioning the focusing optics and beamsplitter relative to the viewer such that the pupil of the eye stands in the same relation to the beamsplitter as the focusing optics stands in relation to the beamsplitter to view an image from the image source when light is reflected in the conjugate light path. 